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A More Technical Description
Lowering the iris screw corresponds to increasing the turns ratio n in the
equivalent circuit shown in Figure 5-5. If the iris screw is still further lowered, the cavity becomes overcoupled. The Q therefore decreases and microwaves are reflected back. (See Equation [5-4] through Equation [5-6] and
Figure 5-8.) The lower Q can be seen from the width of the cavity resonance.
Figure 5-8
The mode picture of an overcoupled cavity.
Sometimes, if the iris is even further lowered, we can once again match and
undercouple the cavity. This is not particularly mysterious if you study
Figure 5-9.
Figure 5-9
Two equivalent positions of an iris screw.
The metal stub on the end of the iris screw is in equivalent positions for a
high and low iris screw position. The disadvantage of the lower position is
that the microwaves have to pass through the lossy plastic of the iris screw.
Figure 5-9 also suggests a solution if you are having problems matching your
cavity. If you use a larger iris tip, you can change the effective size of the iris
more, resulting in a greater coupling range.
Larger iris tips are particularly useful for lossy samples. Lossy samples (usually) interact with the electric field of the microwaves to heat the sample. This
effect is similar to heating food in a microwave oven. Examples of lossy samples are polar liquids such as water, sulfuric acid, chloroform, and methanol.
Sometimes, semiconductor samples exhibit such effects. This non-resonant
5-6